What is the process to reenlist after being out for less than 18 months?

Your first hurdle, as it were, is knowing your RE-Code. U.S. Military RE codes, found on military discharge documents (DD-214), define a service member’s future eligibility to enlist or reenlist after discharge or separation from the military.

RE Code 1: May reenlist without issue.
RE Code 2: May reenlist, but restrictions may apply. This code may also mean the circumstances that resulted in the code are no longer applicable.
RE Code 3: May reenlist but may need a waiver.
RE Code 4: Ineligible to reenlist or join another service without an Exception to Policy (ETP) waiver.

Your DD-214 in Block 26 will show what your RE Code is. If there is a 1 there, then you're good to go

It is not reenlisting. You are now a prior service soldier enlisting. There isn't really tricks. You will fall under the business rules. HRC pushes out rules for those coming back to the military. You don't really have the same options as someone joining for the first time. Luckily, the Army is hurting for recruits so you may have some good options. This will depend on your RE Code on your DD214.

Are you still in the IRR? The only tip I can think of is if you're still "in" it's easier to stay "in" than to enlist off the street.